1. Field of the Invention
The present invention relates to a method for converting biomass raw materials to synthesis gas composed chiefly of carbon monoxide and hydrogen.
2. Description of the Prior Art
Many techniques are known for converting biomass into a synthetic gaseous fuel and inert ash residue. Normally, a biomass raw material which can be either organic waste, residues or biomass grown specifically as a chemical feedstock is introduced into a vertical-bed reactor with sufficient air or oxygen at a temperature sufficient to gasify the biomass to yield a combustible gas and an inert ash. In some systems, steam is added to the mixture in the gasifier to form the synthesis gas during pyrolysis of the biomass. For instance, Mallan et al in U.S. Pat. No. 29,312, illustrate a gasification technique in which a mixture of water, carrier gas, char and particulate carbonaceous solids is passed into a pyrolysis reactor where the mixture is heated to a temperature ranging from about 1200.degree. F. to 2500.degree. F. The temperature of the pyrolysis zone is maintained at a level to optimize the yield of organic chemical and fuel values in the material discharged from the pyrolysis zone which is composed of char, volatilized organic fuel and chemical values, product gas and carrier gas. At least a portion of the hot char obtained can be recycled to the pyrolysis zone as a heat source. On the other hand, excess char particles can be degasified to yield a hydrogen rich gas or the char can be briquetted. A disadvantage of the process is that in the pyrolysis of the organic material, a complex product mixture is obtained of solid, condensable gas and non-condensable gas materials. Synthesis gas is not a product of pyrolysis.
The Choi et al reference, U.S. Pat. No. 4,078,973, shows the pyrolysis of particulate organic solid wastes in an oxygen free carrier gas in which hot char is used as a heat source for the pyrolysis. The char is burned in air and the flue gas therefrom heats the particles sent to the pyrolysis unit and the material exiting the pyrolysis zone consists of a complex mixture of gases, pyrolytic oils and char. It is evident, however, that a relatively simple gaseous fuel mixture such as synthesis gas is not produced.
Honnold, Jr., in U.S. Pat. No. 3,039,955, discloses a pyrolysis process in which one of a variety of fossil fuel materials such as coal, oil, shale, lignite, peat, various petroleum fractions and the like is introduced into a pyrolysis reactor in admixture with steam. Also introduced into the reactor is a stream of hot gases containing entrained coke solids. Pyrolysis of the carbonaceous feed or hydrocarbon feed material produces a solid product and a product off-gas containing hydrogen and a complex mixture of the condensable and non-condensable hydrocarbon gases. Synthesis gas consisting of hydrogen and carbon monoxide is not produced by the pyrolysis reaction.
The Caughey reference, U.S. Pat. No. 4,030,895, discloses a technique of synthesizing a combustible gas product from a biomass material. In the method green wood chips are dried and then pyrolyzed in a gasifier in the presence of air. However, the gaseous product produced by the one step pyrolysis process largely consists of carbon monoxide formed by the partial oxidation of carbon. The gas product also contains gaseous components which are formed by the destructive distillation of the biomass material. The process, however, does not produce synthesis gas as either a primary or secondary gaseous product.
Still another reference, Hand, U.S. Pat. No. 3,963,426, shows a technique of gasifying coal and other carbonaceous materials such as lignite, manure, saw dust, or the like to form a synthesis gas product. In the process, the carbonaceous raw material is dried with hot synthesis gas and the dried material is pyrolyzed in a one-step process in the presence of an oxygen bearing gas-steam mixture, wherein the steam is derived from the moisture bearing synthesis gas, to prepare a synthesis gas product. However, the synthesis gas is contaminated with an inert gaseous component such as nitrogen and contains significant quantities of carbon dioxide and other gas components. In another technique of coal gasification, Babbit in U.S. Pat. No. 4,013,428, shows a process for gasifying coal in which a fuel such as hydrogen is burned with air to produce steam. The steam mixes with powdered coal in a gasifier and a synthesis gas product is formed by the reaction of steam with the coal. However, the synthesis gas product contains inert gaseous materials such as nitrogen as well as carbon dioxide. Moreover, the process is directed to the use of coal as a raw material which is significantly different in composition from such cellulose based biomass materials as wood chips, saw dust and the like which yield a product gas of different composition upon gasification than coal.
Generally speaking, prior art techniques of gasifying biomass materials give rise to a product off-gas stream which is a complex mixture of hydrogen, carbon dioxide, methane, carbon monoxide, nitrogen, water vapor, other low molecular weight hydrocarbon gases such as ethane and propane, condensable organic compounds and tar forming materials. While it is apparent that such a generated off-gas could be used as a fuel, serious problems are encountered in the gasifier used in such processes through tar formation and because of the condensation of liquid organic and aqueous phases in the system. Yet another disadvantage of the prior art biomass conversion processes is that the non-condensable gases formed upon gasification are a complex mixture from which it would be very difficult to recover only carbon monoxide and hydrogen by a necessarily complex process. Generally, in the past, both biomass and char have been thermally degraded in the same reactor which is the reason why a complex mixture of gases is produced as the gaseous pyrolysis product. Still another drawback of the conventional conversion processes is that air is generally used to provide the source of oxygen which appears in the product gas. Therefore, nitrogen is necessarily introduced into the gaseous product obtained, and in order to convert the gaseous product to syngas (synthesis gas), nitrogen must be removed.
Considering at this point the gasifiers which have been used in the past for the partial oxidation of biomass materials to a product gas, the gasifiers have been normally designed in a fashion such that thermal decomposition of biomass material occurs initially to yield volatile off-gases and a char. The initial decomposition occurs by the heat provided by partial oxidation of the off-gases. Gasification of the char then occurs by reaction of the char with oxygen to produce a mixture of carbon monoxide and carbon dioxide. However, the temperatures reached during the oxidation process can be high enough to cause slagging of the ash. While the gasifier can be operated under conditions such that not all of the char is oxidized, this leads to significant reduction in the yield of product gas and therefore inefficiency of operation. In this regard, Caughey in U.S. Pat. No. 4,095,958, discloses a retort for the pyrolysis of biomass material wherein it is indicated that the temperature of the grate of the retort must be maintained below the fusion temperature of the ash and this can be accomplished by the expediency of supplying an excess of primary air to the plenum chamber at the lower end of the grate so that it flows upwardly along the downwardly facing side with the surplus air being discharged at the top of the retort. The grate is also provided with a plurality of small openings and fins on the downward facing side of the grate perpendicular to the plane of the underside of the grate which extends into the flow path of the primary air. In view of the problems encountered in the prior art for converting biomass to usable fuels, a need continues to exist for a method by which biomass material can be pyrolyzed and gasified directly to synthesis gas composed of hydrogen and carbon monoxide.